1. Field of the Invention
This invention relates generally to the growth of a biaxially textured metal oxide buffer layer on metal substrates. More particularly, it relates to a sol-gel technique for growing such buffer layers on metal substrates.
2. Background of the Art
Biaxially textured metal oxide buffer layers on metal substrates are potentially useful in electronic devices where an electronically active layer is deposited on the buffer layer. The electronically active layer may be a superconductor, a semiconductor, or a ferroelectric material.
For example, superconducting wire to be used for power transmission lines has a multi-layer composition 10 (FIG. 1). Such deposited conductor systems consist of a metal substrate 11, buffer layer(s) 12, and a superconducting layer 13. The metal substrate, such as Ni, Ag, or Ni alloys, provides flexibility for the wire and can be fabricated over long lengths and large areas. Metal oxide buffer layers, such as LaAlO.sub.3, CeO.sub.2, or yttria-stabilized zirconia (YSZ), comprise the next layer and serve as chemical barriers between the metal substrate and the last layer, the high-temperature superconductor.
To achieve high critical current densities from the wire, it is important that the superconducting material be biaxially oriented and strongly linked. The orientation necessary begins with the texture of the metal substrate which must be maintained in the buffer layer and thus transferred to the superconductor. The conventional processes that are currently being used to grow buffer layers on metal substrates and achieve this transfer of texture are vacuum processes such as pulsed laser deposition, sputtering, and electron beam evaporation. Researchers have recently used such techniques to grow biaxially textured YBa.sub.2 Cu.sub.3 O.sub.x (YBCO) films on metal substrate/buffer layer samples that have yielded critical current densities between 700,000 and 10.sup.6 A/cm.sup.2 at 77.degree. K. (A. Goyal, et al., "Materials Research Society Spring Meeting, San Francisco, Calif., 1996; X. D. Wu, et al., Appl. Phys. Lett. 67:2397, 1995). One drawback of such vacuum processes is the difficulty of coating long or irregularly-shaped substrates, and the long reaction times and relatively high temperatures required.
A further consideration during the fabrication process is the undesirable oxidation of the metal substrate (for example, when using Ni). If the Ni begins to oxidize, the resulting NiO will likely grow in the (111) orientation regardless of the orientation of the Ni (J. V. Cathcart, et al., J. Electrochem. Soc. 116:664, 1969). This (111) NiO orientation adversely affects the growth of biaxially textured layers and will be transferred, despite the substrate's original orientation, to the following layers.
Thus, it would be advantageous to have a process which avoids the above disadvantages of vacuum-based processes.